1. Field of the Invention
This invention relates to a solid-state image pickup apparatus, and more particularly to a solid-state image pickup apparatus from which signal charge for two lines can be read out at a time.
2. Description of the Prior Art
A solid-stage image pickup apparatus is already known and disclosed, for example, in Japanese Patent Laid-Open Application No. 62-92587 which includes a plurality of light sensitive sections disposed two-dimensionally for individual picture elements in horizontal and vertical directions, a vertical transferring section including a plurality of vertical shift registers for transferring signal charge, read out from the light sensitive sections, in the vertical direction by two line for one horizontal blanking period, and a horizontal transferring section including a pair of horizontal shift registers connected in parallel to each other to output ends of the vertical transferring section for transferring two-line of signal charges, supplied from the vertical transferring section, at a time in the horizontal direction, and for reading out two-line of signal charges at a time. Here, a row of picture elements in the horizontal direction is referred to as line.
Where a solid-state image pickup apparatus is employed, for example, for a color video camera, a predetermined color filter is provided for each of a plurality of light sensitive sections so that each picture element may have a predetermined spectral sensitivity characteristic in order to obtain a color signal. Such color filters are arranged, in some conventional solid-state image pickup apparatus, in such a coding pattern that a color filter A and another color filter C are disposed alternately in each odd-numbered line while a further color filter B and a still further color filter D are disposed alternately in each even-numbered line as shown in FIG. 2.
With such solid-state image pickup apparatus of the type from which signal charge of two lines is read out at a time as described above, transfer in the vertical direction takes place, in each transferring operation, for two lines for part of a horizontal blanking period, but for any other period, signal charge stops in the vertical shift registers. Accordingly, for a portion of each horizontal blanking period in which vertical transfer is not performed, the solid-state image pickup apparatus is liable to be considerably influenced by a smear caused by light admitted in through openings in which the color filters are disposed. Thus, light may be introduced into a vertical shift register through a color filter A or C in an odd-numbered line, but through a color filter B or D in an even-numbered line. Consequently, the amount of such smear will be different for each line.
Here, reading of signal charge in an interlacing fashion is examined. Normally, in a first field, signal charge a from a picture element of a color filter A is transferred, upon transferring operation after it has read out into a vertical shift register, by a two step distance in the vertical shift register as seen from FIG. 4(A). In this instance, a smear component originating from light incident through the color filter A of the picture element for which the signal charge a has been read out is dominant as a smear component included in the signal charge. Meanwhile, in a second field, signal charge a of a picture element of the color filter A is transferred, due to interlacing reading, by a one step distance immediately after it has been read into the vertical shift register, and then transferred, upon each transferring operation, by a two step distance in the vertical shift register. In this instance, although a smear component originating from light incident through the color filter A is dominant, before reading, as a smear component included in the signal component a, since the signal charge a is transferred, after reading, by a two step distance from the position corresponding to a color filter B, a smear component originating from light incident through the color filter B is mixed principally, and consequently, such two smear components are finally included in a mixed condition in the signal charge being transferred.
In this manner, such conventional solid-state image pickup apparatus of the type which reads two lines at a time as described above is disadvantageous, in reading signal charge in an interlacing fashion, in that
(1) an amount of smear included in signal charge for a same picture element is different between first and second fields, and that
(2) in a second field, the level of a smear component is different whether a point light source is located at an upper location or a lower location of a screen.
The second disadvantage arises from the fact that, since a smear component is overlapped with signal charge which has passed a location around a point light source, when considered from a second field, a smear component at an upper location of a screen originates from light incident through the color filter before reading, but a smear component at a lower location of the screen originates from light incident through the color filter B after reading.
As a result, a smear component may be colored extraordinarily after signal processing, or an amount of smear may look different at upper and lower locations of a screen. Such phenomenon causes a problem that, particularly when correction to reduce an amount of smear is to be made, a circuit for such correction is complicated, and so forth.